1. Field of the Invention
The present invention relates to an electrode for an electric double layer capacitor.
2. Disclosure of the Related Art
Electrochemical capacitors, a typical example of which is an electric double layer capacitor, are expected, for example, as backups for power sources of portable devices (small-sized electric devices) and others, auxiliary power sources for electric automobiles or hybrid automobiles, and the like. Various investigations for improving the performance of the capacitors have been made.
As portable devices and others have been becoming smaller and lighter in recent years, it has been particularly required to make an electric double layer capacitor smaller and lighter while the capacitor keeps sufficient performances required for devices on which the capacitor is to be mounted. In other words, an improvement in energy density per unit mass of an electric double layer capacitor, and an improvement in energy density per unit volume thereof have been required. Consequently, electrodes used in the electric double layer capacitor have been required to be made smaller and lighter.
As the electrodes, which are used in electric double layer capacitors, there are known electrodes fabricated by making fibrous activated carbon in a felt form and then vapor-depositing or bonding a current collector made of aluminum or the like to one side of the felt-form activated carbon, electrodes fabricated by kneading granular activated carbon together with a binder such as tetrafluoroethylene or the like, forming the kneaded material into a sheet form, and vapor-depositing or bonding a current collector to one side of the sheet, and other electrodes.
However, in the above-described conventional electrodes, which are fabricated by vapor-depositing or bonding a current collector to activated carbon formed in a felt or sheet form, physical adhesiveness between the activated carbon and the current collector is insufficient, and a decrease in the internal resistance is also insufficient. Additionally, there is a limit to an improvement in electrode characteristics. It is therefore difficult to make the electrodes smaller and lighter while the electrode characteristics are sufficiently maintained.
There is also suggested, for example, a tabular electrode fabricated by forming a kneaded product composed of a binder and a porous material such as activated carbon into a sheet form so as to yield a polarizable electrode (electrode layer), and then bonding the resultant electrode to a current collector through an intermediate layer composed of carbon black and a binder. In the electrode, the adhesiveness between the electrode layer and the current collector is improved and the contact resistance therebetween is decreased (see, for example, Japanese Laid-Open Patent Publication No. 2000-208373, Japanese Laid-Open Patent Publication No. 2001-284184, Japanese Laid-Open Patent Publication No. 2002-75805, and Japanese Laid-Open Patent Publication No. 2002-50546).
However, in each of the above-mentioned publications, an electrode layer is beforehand formed in a sheet form, and this sheet-form electrode layer is bonded to a current collector through an intermediate layer to fabricate an electrode. It is therefore necessary that the electrode layer has a thickness satisfying a mechanical strength for maintaining the shape of the sheet. Thus, it is difficult to make the thickness of the electrode layer smaller.
And, Japanese Laid-Open Patent Publication No. 11-238654 (1999) discloses a process of joining an electrode made mainly of a carbonous material to a surface of a current collector through an electrically conductive adhesive agent comprising, as a binder component, an epoxy resin containing an aliphatic epoxide as a reactive diluting agent, and then heating and curing the adhesive agent to fabricate an electrode. The used adhesive agent is thermosetting.